Techniques for depositing coating on ceramic substrate

ABSTRACT

A method of coating a substrate, the method including depositing mullite on the substrate during a first time period via thermal spraying to form a first layer, the mullite comprising mullite powder formed via at least one of a fused plus crush or sinter plus crush process; and depositing a second material on the first layer to form a second layer, wherein the substrate is at a temperature less than approximately 50 degrees Celsius at approximately a beginning of the first time period. In some embodiments, the method may further include depositing silicon to form a silicon bond layer between the substrate and mullite layer.

This application claims priority from U.S. Provisional Application Ser.No. 61/231,510 entitled “TECHNIQUES FOR DEPOSITING COATING ON CERAMICSUBSTRATE,” filed Aug. 5, 2009, the entire content of which isincorporated herein by reference.

TECHNICAL FIELD

The disclosure relates to coatings for high-temperature mechanicalsystems, such as gas turbine engines, and more particularly to coatingsincluding one or more mullite layers.

BACKGROUND

The components of high-temperature mechanical systems, such as, forexample, gas-turbine engines, must operate in severe environments. Forexample, hot section components of gas turbine engines, e.g., turbineblades and/or vanes, exposed to hot gases in commercial aeronauticalengines may experience surface temperatures of greater than 1,000° C.Furthermore, economic and environmental concerns, e.g., the desire forimproved efficiency and reduced emissions, continue to drive thedevelopment of advanced gas turbine engines with higher gas inlettemperatures. As the turbine inlet temperature continues to increase,there is a demand for components capable of operating at such hightemperatures.

Components of high-temperature mechanical systems may include ceramicand/or superalloy substrates. Coatings for such substrates continue tobe developed to increase the operating capabilities of such componentsand may include thermal barrier coatings (TBC) and environmental barriercoatings (EBC). In some examples, thermal barrier coatings (TBC) may beapplied to substrates to increase the temperature capability of acomponent, e.g., by insulating a substrate from a hot externalenvironment. Further, environmental barrier coatings (EBC) may beapplied to ceramic substrates, e.g., silicon-based ceramics, to provideenvironmental protection to the substrate. For example, an EBC may beapplied to a silicon-based ceramic or ceramic composite substrate toprotect against the recession of the ceramic substrate resulting fromoperation in the presence of water vapor in a high temperaturecombustion environment. In some cases, an EBC may also function as a TBCbased on low thermal conductivity values of the EBC, although a separatecompatible TBC may also be added to a substrate in addition to an EBC tofurther increase the temperature capability of a component.

SUMMARY

In general, the disclosure relates to techniques for depositing one ormore materials on a substrate to form layers that make up a coating.Such techniques may be applicable to the depositions of a coating onceramic or ceramic composite substrates, which in most cases mayfunction as an environmental barrier coating (EBC) e.g., when applied toceramic components of high temperature mechanical systems. In somecases, an EBC may include a silicon bond layer, an intermediate layercontaining mullite (3Al₂O₃.2SiO2) and an outer layer. Themullite-containing layer may be formed by depositing mullite powder onthe ceramic substrate. In some embodiments, mullite powder may bedeposited on a ceramic substrate via thermal spraying, e.g., air plasmaspraying, to form a layer including the deposited mullite. The outerlayer may be formed by depositing a second material on themullite-containing layer. In some embodiments, the second material mayinclude barium strontium aluminum silicate (BSAS) or one or morerare-earth silicates, such as Yb-monosilicate or Yb-disilicate.

According to some embodiments of the disclosure, mullite powder may bedeposited on a ceramic substrate via thermal spraying to form amullite-based layer of an EBC in a low temperature environment. Forexample, a ceramic substrate may be at a substantially uniformtemperature of less than 50 degrees Celsius at least at the beginning ofthe mullite deposition process. Such a temperature limit may correspondto conditions achievable without providing supplemental heat to aceramic substrate, as may be the case in a thermal spraying while thesubstrate is in a furnace and/or the use of back side heating during themullite deposition process. Despite the low temperature environment inwhich the thermal spraying of the mullite powder takes place, themullite layer formed from the deposition may perform substantially thesame or even better than mullite layers formed by depositing mullite viathermal spraying in a higher temperature environment, e.g., thoseassociated with the use of furnace deposition and/or back-side heatingof the substrate.

As will be described in further detail below, mullite powder that hasbeen manufactured by a fused or sinter plus crush process, e.g., ratherthan that manufactured by a spray granulation process, may allow for thethermal spraying of mullite powder in a low temperature environment toform a mullite layer that perform substantially the same or even betterthan mullite layers formed via thermal spraying of mullite powder in ahigher temperature environment. For example, mullite powder manufacturedby a fused plus crush or sinter plus crush process may be thermallysprayed on ceramic substrates starting at a time when the substratetemperatures is less than 50 degrees Celsius to form amullite-containing layer that performs adequately at high temperatureswithout exhibiting substantial cracking or delamination during thermalcycling, even without heat treating the mullite layer after itsformation.

Techniques for forming an EBC that includes a silicon bond layer appliedbetween a ceramic substrate and a mullite layer are also described. Sucha silicon bond layer may be formed by depositing appropriate siliconmaterial on a ceramic substrate prior to the deposition of the mullitematerial via thermal spraying. In some embodiments, the silicon bondlayer may undergo a high temperature heat treatment for a relativelyshort amount of time, e.g., approximately 1 hour at about 1200 degreesCelsius, prior to the deposition of the mullite material on the siliconbond layer via thermal spraying. Such a technique may promote adhesionbetween the silicon bond layer and the ceramic substrate to enhance theadhesion of the EBC system to the ceramic substrate.

In one embodiment, the disclosure is directed to a method of coating asubstrate comprising depositing mullite on the substrate during a firsttime period via thermal spraying to form a first layer, the mullitecomprising mullite powder formed via at least one of a fused plus crushor sinter plus crush process; and depositing a second material on thefirst layer to form a second layer, wherein the substrate is at atemperature less than approximately 50 degrees Celsius at approximatelya beginning of the first time period.

In another embodiment, the disclosure is directed to a method of coatinga substrate, the method comprising depositing silicon on the substrateto form a silicon layer; heat treating the silicon layer; depositingmullite on the silicon layer via thermal spraying to form a first layersubsequent to the heat treatment of the silicon layer, the mullitecomprising mullite powder formed via at least one of a fused plus crushor sinter plus crush process; depositing a second material on the firstlayer to form an intermediate layer; and depositing a third material onthe intermediate layer to form a third layer.

In another embodiment, the disclosure is directed to a method of coatinga substrate, the method comprising depositing mullite on the substratevia thermal spraying to form a first layer; and depositing a secondmaterial on the first layer via thermal spraying to form a second layer,the second material comprising at least one of barium strontium aluminumsilicate (BSAS) or a rare-earth silicate, wherein the substrate is at atemperature of less than 150 degrees Celsius when the second material isfirst deposited.

In another embodiment, the disclosure is directed to a method of coatinga substrate, the method comprising depositing silicon on the substrateto form a silicon layer; heat treating the silicon layer; and depositingmullite on the substrate during a first time period via thermal sprayingto form a first layer, the mullite comprising mullite powder formed viaat least one of a fused plus crush or sinter plus crush process.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional diagram illustrating an example articleincluding an example EBC on a substrate.

FIG. 2 is a flow chart illustrating an example technique for applyinglayers of an EBC on a substrate to generate the example article of FIG.1.

FIG. 3 is a flow chart illustrating another example technique forapplying layers of an EBC on a substrate to generate the example articleof FIG. 1.

FIG. 4 is a cross-sectional diagram illustrating another example articleincluding an example EBC on a substrate.

FIGS. 5A and 5B are cross-sectional photographs of a portion of examplearticles including example EBCs including layers that were deposited atapproximately 1200 degrees Celsius and approximately 25 degrees Celsius,respectively.

FIGS. 6A and 6B are additional cross-sectional photographs of theexample articles of FIGS. 5A and 5B, respectively.

DETAILED DESCRIPTION

In general, embodiments of the disclosure relate to techniques forapplying coatings to a variety of substrates, such as, e.g., ceramicsubstrates. In some embodiments, the applied coatings may function as anEBC on ceramic substrates used for components of high temperaturemechanical system to increase the operating capabilities of componentsof high temperature mechanical systems and extend the componentsdurability. For example, an EBC may be applied to a silicon-basedceramic component to protect against recession of the component causedby the volatilization of silica scale by water vapor in the hightemperature combustion environment.

As will be described below, an EBC may be a multilayer coating includingan outer layer containing one or more rare-earth silicates and/or bariumstrontium aluminum silicate (BSAS) that may be bonded to the ceramicsubstrate via a mullite-containing layer, which may be referred as themullite layer. In some cases, the EBC may include a silicon-based bondlayer provided between the substrate and mullite layer to adhere themullite layer to the substrate and to extend the life of the coating.

The mullite layer of an EBC may be formed by depositing mullite in theform of mullite powder on the surface of a substrate via a thermalspraying process, such as, e.g., plasma spraying. In some examples, themullite powder used may be manufactured using a plasma spray granulationprocess. However, due to the relatively high phase instability ofmullite manufactured by a plasma spray granulation process, the mullitelayer formed via the thermal spraying process may includenon-homogenized mullite in both amorphous and crystalline phases. If themullite layer contains too high a concentration of amorphous phasemullite, then the environmental protection provided by the EBC to thesubstrate may be reduced over the life of substrate. For example, theamorphous non-homogenous mullite in a mullite layer may be prone tocrystalline phase transformations during thermal cycling that inducevolumetric changes within the mullite layer. In some cases, thevolumetric changes may cause cracking and/or delamination of the mullitelayer, which may reduce the extent of environmental protection providedby the EBC to the ceramic component.

To reduce the amount of amorphous mullite in the mullite layer, thethermal spraying process may include heating the ceramic substrate to anelevated temperature within a closed environment (e.g., by heating thesubstrate within a furnace room maintained at approximately 1200 degreesCelsius or providing supplemental heat towards one or more surfaces ofthe substrate) and maintaining the substrate at the elevated temperatureduring the thermal spraying of the mullite powder, which has beenmanufactured by plasma spray granulation technique, to promote thedeposition of crystalline mullite rather than amorphous mullite.Similarly, the substrate may also be maintained at an elevatedtemperature during the deposition of the material used to form the outerlayer, e.g., one or more rare-earth silicates and/or BSAS, to providefor suitable performance of the outer layer and/or bonding to themullite layer.

Additionally or alternatively, a substrate may be heat treated, e.g., ata temperature greater than 1200 degrees Celsius for greater than 10hours, after one or more of these EBC layers have been formed on thesubstrate to homogenize/stabilize the crystalline phase mullite ortransform amorphous mullite to crystalline mullite in the EBC prior toexposing the substrate to thermal cycling to increase the reliability ofthe EBC.

However, while the high temperature application and/or heat treatment ofone or more layers of the EBC can in some cases address the phaseinstability issues, such a high temperature process requirements candramatically increase the cost and relative complexity of applying anEBC to a ceramic component. Furthermore, such process requirements candecrease the flexibility of manufacturing and design of such components.For example, in such cases, the overall size and/or shape of a componentthat the EBC is being applied to may be limited by the relativedimensions of the heat controlled environment, such as the size of thehigh temperature furnace, required to maintain the component at anappropriate elevated temperature during the application of one or moreEBC layers and/or heat treat a coated substrate.

As will be described in greater detail below, embodiments of thedisclosure may include techniques for forming one or more layers of anEBC, including a mullite layer, on a substrate in a low temperatureenvironment by depositing the respective material via thermal sprayingwithout requiring extended high temperature heat treatment of themullite-coated substrate. Despite the low temperature deposition of thematerials that form the respective layers of the EBC, the EBC mayperform the same or similar to that of an EBC with respective layer(s)that have been deposited via thermal spraying on the substrate atelevated temperatures. In particular, the mullite layer may be formed ona substrate by depositing mullite powder manufactured via a fused orsinter plus crush process, e.g., rather than a plasma spray granulationprocess, via thermal spraying, such as, e.g., air plasma spraying, evenwithout substantially heating the substrate in conjunction with thedeposition and/or heat treating the mullite-coated substrate. Despitethe low temperature application, such a mullite layer may not exhibitsubstantial cracking or delamination from the substrate even afterundergoing thermal cycling.

FIG. 1 is a cross-sectional diagram illustrating an example article 10,which may be used in a high temperature mechanical system. Article 10includes EBC 14 applied on substrate 12. As shown, EBC 14 is amultilayer coating that includes bond layer 16, intermediate layer 18and outer layer 20. In general, EBC 14 may provide environmentalprotection to allow article 10 to operate in severe environments, e.g.,by preventing recession of substrate 12 in a high temperature combustionenvironment.

Substrate 12 may be a component of a high temperature mechanical system,such as, e.g., a hot section component of a gas turbine engine. Examplesof such components may include, but are not limited to, turbine blades,blade tracks, combustion liners, and the like. Substrate 12 may includesilicon-containing ceramics, such as, e.g., silicon carbide (SiC),silicon nitride (Si₃N₄), composites having a SiC or Si₃N₄ matrix,silicon oxynitride, and aluminum oxynitrides; an silicon containingmetal alloy, such as molybdenum-silicon alloys (e.g., MoSi₂) andniobium-silicon alloys (e.g., NbSi₂); and an oxide-oxide ceramic.

Substrate 12 may include a matrix, such as, e.g., a ceramic matrixcomposite (CMC), which may include any useful ceramic matrix material,including, for example, silicon carbide, silicon nitride, alumina,silica, and the like. The matrix may further include any desired fillermaterial, and the filler material may include a continuous reinforcementor a discontinuous reinforcement. For example, the matrix may bereinforced with ceramic fibers, whiskers, platelets and chopped orcontinuous fibers.

The filler composition, shape, size, and the like may be selected toprovide the desired properties to the matrix. For example, in someembodiments, the filler material may be chosen to increase the toughnessof a brittle ceramic matrix. In other embodiments, the filler may bechosen to provide a desired property to the matrix, such as thermalconductivity, electrical conductivity, thermal expansion, hardness, orthe like.

In some embodiments, the filler composition may be the same as thematrix material. For example, a silicon carbide matrix may surroundsilicon carbide whiskers. In other embodiments, the filler material mayinclude a different composition than the matrix, such as mullite fibersin an alumina matrix, or the like. In one embodiment, a CMC may includesilicon carbide continuous fibers embedded in a silicon carbide matrix.

EBC 14 may include bond layer 16, which in the example of FIG. 1 isapplied directly on substrate 12. Bond layer 16 may include silicon,e.g., in the form of high purity, plasma sprayable grade silicon powder,and may be formed by depositing the appropriate silicon material onsubstrate 12 via one or more suitable thermal spraying processes, suchas, e.g., plasma spraying. In some cases, bond layer 16 may optionallybe included in EBC 14 to promote adherence of intermediate layer 18and/or outer layer 20 to substrate 12. While EBC 14 is shown with bondlayer 16, in some embodiments, EBC 14 may not include bond layer 14. Forexample, intermediate layer 18 may be formed directly on substrate 12rather than being separated by bond layer 16.

Bond layer 16 may formed at any suitable thickness, such as a thicknessthat allows EBC 14 to provide environmental protection to article 10 asdescribed herein. For example, in one embodiment, bond layer 16 may havethickness in the range of approximately 0.2 mils to approximately 5mils. In some embodiments, bond layer 16 may have thickness in the rangeof approximately 0.5 mils to approximately 5 mils, such as approximately2 mils to approximately 4 mils or approximately 1 mil to approximately 4mils.

EBC 14 may also include intermediate layer 18, which may be provided ontop of bond layer 16 and/or substrate 12. Intermediate layer 18 mayinclude mullite manufactured by a fused or sinter plus crushedtechnique, at least some of which is in the crystalline phase, and maybe formed by depositing mullite powder on substrate 12 via one or moresuitable thermal spraying processes, including plasma spraying. In someembodiments, intermediate layer 18 may additionally include greater thanapproximately 50 percent by weight crystalline mullite, such as, e.g.,approximately 60 percent by weight crystalline mullite or 80 percent byweight crystalline mullite. In some examples, intermediate layer 18 mayinclude approximately 100 percent by weight crystalline mullite.

In some embodiments, the balance of intermediate layer 18 other thanthat of the mullite may include an amount of BSAS, e.g., in cases inwhich outer layer 20 includes BSAS. For example, intermediate layer 18may include up to approximately 80 percent by weight BSAS, such as,e.g., approximately 40 percent by weight BSAS or approximately 20percent by weight BSAS. In other embodiments, the balance ofintermediate layer 18 may include an amount of rare-earth silicate,e.g., in cases in which outer layer 20 includes rare-earth silicate. Forexample, intermediate layer 18 may include up to approximately 80percent by weight rare-earth silicate, such as, e.g., approximately 40percent by weight rare-earth silicate or approximately 20 percent byweight rare-earth silicate.

The mullite material may be deposited on bond layer 16 and substrate 12via a thermal spraying process to form intermediate layer 18 withoutmaintaining or providing substrate 12 at an elevated temperaturerelative the thermal spraying process and/or heat treating intermediatelayer 18 after the mullite has been thermally sprayed. Despite therelatively low temperature application, intermediate layer 18 may stillperform the same or similar to that of a mullite layer deposited at hightemperature and/or that has undergone heat treatment. In particular, themullite layer formed via the deposition of mullite powder manufacturedvia a fused or sinter plus crush process may not exhibit undesirablevolumetric changes during thermal cycling that may cause EBC 14 to fail,e.g., due to cracking or delamination, even though the substrate was notmaintained at an elevated temperature during the thermally sprayingprocess or heat treated after intermediate layer 18 was formed. In someembodiments, intermediate layer 18 may include greater thanapproximately 50 percent by weight crystalline mullite, such as, e.g.,greater than approximately 80 percent by weight crystalline mullite,even without thermal spraying the mullite powder on substrate 12 viathermal spraying or heat treating substrate 12 after the formation ofintermediate layer 18.

As previously mentioned, such a process may be enabled by using amullite powder for thermal spraying that has been generated via a fusedplus crush process and/or sinter plus crush process rather than via aplasma spray granulation process, for example. In some cases, mullitepowder generated via a fused or sinter plus crushed process may allowfor deposition of the mullite powder via thermal spraying at arelatively lower temperature than mullite powder generated via spraygranulation. Although not wishing to be limited by theory, it isbelieved that mullite powder generated via a fused or sinter plus crushprocess may contain relatively high concentration of stoichiometricamorphous phase mullite in the powder material. Conversely, mullitepowder generated via spray granulation may include a relatively highconcentration of non-stoichiometric or non-homogenous phase mullitepowder. The volumetric changes associated with conversion ofstoichiometric amorphous phase mullite to crystalline mullite in a hightemperature environment is less than the volumetric changes associatedwith the phase conversion of non-stoichiometric amorphous phase mullitedue to increased homogenity. In some cases, the volumetric changesassociated with the conversion of stoichiometric amorphous phase mullitemay not be great enough to cause delamination of an EBC due to thethermal expansion of the mullite layer in high temperatures, while thevolumetric changes associated with the conversion of non-stoichiometricamorphous phase mullite may be great enough to cause delamination of anEBC due to the thermal expansion of the mullite layer in hightemperatures.

Intermediate layer 18 may be formed at any suitable thickness, such as athickness that allows EBC 14 to provide environmental protection toarticle 10 as described herein. For example, one embodiment,intermediate layer 18 may have thickness in the range of approximately 1mil to approximately 7 mils. In still another embodiment, intermediatelayer 18 may have thickness in the range of approximately 4 mils toapproximately 6 mils. In some embodiments in which EBC 14 includes bondlayer 16 and intermediate layer 18, the ratio of layer thickness betweenbond layer 16 and intermediate layer 18 may range from approximately 0.1to approximately 1.5 such as, e.g., from approximately 0.2 toapproximately 1.0.

EBC 14 may also include outer layer 20. In the example of FIG. 1, outerlayer 20 has been formed directly on intermediate layer 16 such that itis adhered to substrate 12 via intermediate layer 18 and bond layer 16.Outer layer 20 may include one or more components selected to provideenvironmental protection for substrate 12 in combination with bond layer16 and intermediate layer 18 as described herein. In some embodiments,outer layer 20 may include one or more rare-earth silicates, such as,e.g., ytterbium mono-silicate or ytterbium di-silicate. Additionally oralternatively, outer layer 20 may include BSAS, e.g., in situations inwhich the operating temperature of the component is below approximately2400 degrees Fahrenheit.

The material selected for outer layer 20 may be deposited onintermediate layer 18 via a thermal spraying process to form outer layer18. As will be described in further below with respect to FIG. 2, insome embodiments, the outer layer material may be thermally sprayed onintermediate layer 18 to form outer layer 20 while substrate 12 ismaintained at substantially uniform temperature that is less than 150degrees Celsius, such as, e.g., less than 50 degrees Celsius. In somecases, this may include depositing the outer layer material, e.g., BSASor one or more rare-earth silicates, via thermal spraying on substrate12 while the substrate is provided at a temperature that isapproximately the same as that of substrate 12 when the mullite materialwas initially deposited via thermal spraying to form intermediate layer18. Despite the relatively low deposition temperature, it may bepossible to form outer layer 20 including one or more rare earthsilicates or BSAS that is adequately bonded to intermediate layer 18while still preventing coating delamination in outer layer 20. In thismanner, substrate 12 may be coated with intermediate layer 18 and outerlayer 20 via thermal spraying without having to provide additional heatto substrate 12 during the application process of EBC 14.

Outer layer 20 may be formed at any suitable thickness, such as athickness that allows EBC 14 to provide environmental protection andthermal barrier to article 10 as described herein. For example, in oneembodiment, outer layer 20 may have thickness in the range ofapproximately 2 mils to approximately 15 mils. In still anotherembodiment, outer layer 20 may have thickness in the range ofapproximately 4 mils to approximately 12 mils.

FIG. 2 is a flow chart illustrating an example technique for applyinglayers 16, 18, 20 of EBC 14 on substrate 12 to generate article 10 ofFIG. 1. As indicated in FIG. 2, uncoated substrate 12 may be provided ata temperature of less than approximately 50 degrees Celsius (22). Insome cases, substrate 12 may be undergo one or more preparation stepsprior to the being coated with one or more of the layers describedherein. For example, substrate may be grit blasted with a suitablematerial, e.g., aluminum oxide and the like, to prepare the substratesurface for coating. In any case, appropriate silicon material may bedeposited of substrate 12 at via any suitable process, including thermalspraying, to form bond layer 16. In some embodiments, the deposition ofthe silicon material may begin when substrate 12 is at a temperature ofless than 50 degrees Celsius (24).

After bond layer 16 has been formed on substrate 12, mullite powder thathas been manufactured via a fused plus crushed and/or sinter plush crushprocess may be deposited on bond layer 16 via thermal spraying to formintermediate layer 18(26). The deposition of the mullite powder on thesubstrate may begin even when the substrate is at a substantiallyuniform temperature of less than 50 degrees Celsius (26).

After intermediate layer 18 has been formed on substrate 12, the outerlayer material, e.g., one or more rare earth silicates or BSAS, may bedeposited on intermediate layer 18 via thermal spraying to form outerlayer 18. Such a deposition process may also begin even when thesubstrate is at temperature of less than 50 degrees Celsius (28). Inthis manner, the respective layers 16, 18, 20 of EBC 14 may be appliedto substrate 12 in a relatively low temperature air environment whilestill forming a suitable coating that provides environmental protectionto substrate in a high temperature combustion environment.

As illustrated by the example of FIG. 2, in some embodiments, one ormore layers of EBC 14 may be deposited on substrate 12 via thermalspraying even when the temperature of substrate 22 is at a relativelylow, which may include temperatures realized without purposefullyelevating the temperature of the substrate above that generallyexhibited under normal conditions. For example, in FIG. 2, each of bondlayer 14, intermediate layer 18, and outer layer 20 may be formed onsubstrate via thermal spraying when the substrate at a substantiallyuniform temperature of less than approximately 50 degrees Celsius. Sucha temperature limit may generally correspond to the maximum temperaturethat may be naturally exhibited in a space without providing substantialsupplemental heat to the space beyond that provided by conventionaldevice or systems that may be used to control the temperature of a room,e.g., from a heating, ventilation and air conditioning (HVAC) system. Inparticular, the low temperature process may be achieved withoutrequiring substrate to be heated in a furnace room or purposefullyproviding supplemental heat directed to substrate 12 for purposes ofheating all or portions of substrate 12 during the deposition process.

Depending in part on the environment that the layer deposition processis undertaken, e.g., the natural temperature of the room at which thedeposition process is undertaken, substrate 12 may be at a temperatureless than approximately 50 degrees Celsius at the beginning of thedeposition of one or more of layer 16, 18, and 20, which includes thebeginning of the deposition of the mullite powder on the substrate (26),as previously described. For example, substrate 12 may be at atemperature of less than approximately 40 degrees Celsius at thebeginning of the mullite powder deposition, such as, e.g., betweenapproximately 15 degrees Celsius and approximately 35 degrees Celsius.In some embodiments, substrate 12 is not be substantially heated abovethe temperature of the surrounding space in which the thermal sprayingof the layer material is being performed, wherein the ambienttemperature of the surrounding space, e.g., the room in which thethermal spraying process is performed, is less than approximately 50degrees Celsius. In some embodiments, the ambient temperature of thesurrounding space may be between approximately 10 degrees Celsius and 40degrees Celsius, such as, e.g., between approximately 15 degrees Celsiusand 30 degrees Celsius.

Despite the relatively low temperatures of the substrate during thermalspraying, intermediate layer 18 may contain a concentration ofstoichiometric and/or homogenous amorphous and crystalline mullitephases such that cracking and/or delamination of intermediate layer 18is prevented during thermal cycling. In this manner, one or more oflayers 16, 18, 20 may be formed via thermal spraying of mullite powderwithout having to provide additional heat to substrate 12 to atemperature substantially greater than that of the space

While substrate 12 may be at a temperature of less than 50 degreesCelsius at the beginning of the deposition of the materials of layers16, 18, and 20, it is recognized that the local temperature of certainportions of substrate 12 may be increased above approximately 50 degreesCelsius, including local temperature greater than or equal to that of 50degrees, at periods throughout the overall deposition time of therespective layer material. For example, during thermal spraying ofmullite powder on substrate 12 to form intermediate layer 18, thedeposition surface of substrate 22 may reach temperatures greater thanor equal to 50 degrees Celsius because of elevated temperature of thematerial being deposited on the surface of substrate 22. However, whilethe substrate temperature may increase above the temperature at thebeginning of the mullite deposition, it is as a result from the heatprovided by the thermal spraying process rather than by a supplementalsource, as would be the case in a furnace room or with additional heatdirected to substrate 12. Even with the additional heat from the thermalspraying process, in some embodiments, substrate 12 may be maintained ata temperature of less than 200 degrees Celsius, such as, for example,between approximately 15 degrees Celsius and approximately 200 degreesCelsius or between approximately 20 degrees Celsius and approximately150 degrees Celsius, throughout the deposition of the mullite materialvia thermal spraying.

The beginning temperature of substrate 12 relative the deposition of oneor more of the layer materials may be achieved via any suitable method.Substrate 12 may be at a temperature of less than approximately 50degrees Celsius by simply allowing the temperature of substrate 12 to besubstantially equal to that of the room temperature of the surroundingspace, assuming that that surrounding space is less than approximately50 degrees Celsius. This may allow EBC 12, and intermediate layer 18, inparticular, to be formed on substrate 12 via a thermal spraying processwithout having to undertake any additional steps to heat substrate 12during the deposition and/or heat treat substrate 12 after being coatedwith EBC 14

FIG. 3 is a flow chart illustrating an example technique for applyinglayer 16, 18, 20 of EBC 14 on substrate 12 to generate article 10 ofFIG. 1. The example of FIG. 3 includes a heat treatment step after bondlayer 16 has been formed on substrate 12 but before intermediate layer18 has been deposited via thermal spraying. As will be explained below,such an example technique may be useful in situations in which thecoefficient of thermal expansion of two or more individual layers ofcoating 14 are not approximately equal, e.g., differ by approximately 10percent or greater. However, the examples are not limited as such.

Similar to that of the example of FIG. 2, silicon material may bedeposited on uncoated substrate 12 to form bond layer 16 of EBC 14 (32).The appropriate silicon material may then be deposited at via anysuitable process, including thermal spraying, to form bond layer 16 andmay be deposited in some cases when the substrate is at a temperature ofless than 50 degrees Celsius at the starting of the bond layerdeposition (30).

Unlike the example of FIG. 2, after bond layer 16 has been formed,substrate 12 and bond layer 16 may undergo diffusion heat treatmentprior to the deposition of the mullite powder (34). Such a heattreatment may include exposing substrate 12 and bond layer 16 to arelatively high heat environment, e.g., within a furnace, for arelatively short amount of time. In some embodiments, the heat treatmentstep may include placing substrate 12 and bond layer 16 in anenvironment at a temperature between approximately 800 degrees Celsiusand approximately 1250 degrees Celsius for an amount of time betweenapproximately 0.2 hours and approximately 4 hours. For example, in someembodiments, heat treatment step may include placing substrate 12 andbond layer 16 in an environment at a temperature between approximately1100 degrees Celsius and approximately 1225 degrees Celsius for anamount of time between approximately 0.5 hours and approximately 2hours.

After undergoing diffusion heat treatment, mullite powder may bedeposited on bond layer 16 via thermal spraying to form intermediatelayer 18 (36) and outer layer material may be deposited on intermediatelayer 18 via thermal spraying to form outer layer 20 (38), as describedpreviously with respect to FIG. 2. Both intermediate layer 18 and outerlayer 20 may be formed on substrate 12 via plasma spraying withoutplacing substrate 12 at a high temperature environment, e.g., at atemperature greater than 50 degrees Celsius.

The inclusion of the diffusion heat treatment step may serve to increasethe adhesion of EBC 14 to substrate 12, especially in cases in which thecoefficient of thermal expansion of the layers of EBC 14 are not thesubstantially the same. In some embodiment, substrate 12 and bond layer16 may undergo diffusion heat treatment as described above when thecoefficient of thermal expansions of layers 16, 18, 20 differ by morethan 5 percent, such as e.g., more than 10 percent, to increase theadhesion of EBC 14 to substrate 12, even when intermediate layer 18 andouter layer 20 are formed by deposition of the respective materials viathermal spraying without maintaining substrate 12 at an elevatedtemperature, e.g., at a temperature greater than 50 degrees Celsius.

FIG. 4 is a cross-sectional diagram illustrating another example article39, which may be used in a high temperature mechanical system. As shown,example article 39 includes substrate 40, which is substantially thesame or similar to that of substrate 12 of FIG. 1, and EBC 42 applied onsubstrate 12. EBC 42 includes bond layer 44, first intermediate layer46, and outer layer 50, which may be substantially the same or similarto that of bond layer 16, intermediate layer 18 and outer layer 20,respectively, of EBC 14 of FIG. 1.

Unlike the embodiment shown in FIG. 1, EBC 42 includes secondintermediate layer 48 provided between outer layer 50 and firstintermediate layer 46. Second intermediate layer 48 may function toreduce the strain on the interface of outer layer 50 and firstintermediate layer 46 during thermal cycling for cases in which firstintermediate layer 46 and outer layer 50 exhibit different coefficientsof thermal expansion. In some examples, second intermediate layer 48 mayprovide chemical compatibility and/or thermal expansion transitionbetween first intermediate layer 46 and outer layer 48.

The structure and composition of second intermediate layer 48 may vary,and may be selected based on one or more factors. With reference to FIG.4, second intermediate layer 46 may be a single layer or include aplurality of sublayers. In some embodiments, second intermediate layer46 may have a substantially uniform composition throughout, while inother embodiments second intermediate layer 46 may be compositionallygraded based on the composition of the adjacent layers.

In some embodiments, second intermediate layer 48 may include at leastone of alumina and/or mullite. The composition of second intermediatelayer 48 may be selected based on the composition of first intermediatelayer 46 and/or outer layer 50. For example, the composition of firstintermediate layer 46 and outer layer 50 may generally dictate thecoefficient of thermal expansion for the respective layers. Secondintermediate layer 48 may include one or more components which allow forsecond intermediate layer 48 to have a coefficient of thermal expansionthat is in between that of the coefficients of thermal expansion offirst intermediate layer 46 and outer layer 50. In some embodiments,second intermediate layer 48 may have a coefficient of thermal expansionthat is within approximately 10 percent or less than the coefficient ofthermal expansion of first intermediate layer 46 and/or outer layer 50.

In this manner, the difference in thermal expansion between firstintermediate layer 46 and outer layer 50 may be tempered by the thermalexpansion of second intermediate layer 48 when configured as shown inFIG. 4. In some embodiments, alternately or additionally, thecomposition of second intermediate layer 48 may be selected to providesuitable adhesion between second intermediate layer 48 and firstintermediate layer 46, and also between second intermediate layer 48 andouter layer 50.

In some embodiments, second intermediate layer 48 may include one ormore components of both first intermediate layer 46 and outer layer 50.For example, when first intermediate layer 46 includes mullite and outerlayer 50 includes component ytterbium di-silicate, second intermediatelayer 48 may include a mixture of mullite and component ytterbiumdi-silicate. The mixture may include an approximately equal amount ofthe components of first intermediate layer 46 and outer layer 50, or mayinclude any other desired mixture or proportion of components from firstintermediate layer 46 and outer layer 50.

Second intermediate layer 48 may be applied as a separate layer fromfirst intermediate layer 46 and outer layer 50. For example, mullitepowder may be deposited first via thermal spraying to form firstintermediate layer 46, as described herein. The desired mixture of thefirst intermediate layer components, e.g., mullite, and the outer layercomponents may then be mixed and deposited on first intermediate layer46 via thermal spraying to form second intermediate layer 48, followedby application of outer layer 50 on second intermediate layer 48, aspreviously described. Similar to that of outer layer 50 and firstintermediate layer 46, second intermediate layer 48 may be initiallydeposited on first intermediate layer 46 when the temperature ofsubstrate 40 is less than approximately 50 degrees Celsius.

Additionally, second intermediate layer 48 may include more than onesublayers (not shown). In some embodiments, a second intermediate layerhaving one or more sublayers may allow for the interface between firstintermediate layer 46 and outer layer 50 to be compositionally graded.Such compositional grading may reduce the strain on the interface ofouter layer 50 and first intermediate layer 46 during thermal cyclingfor cases in which first intermediate layer 46 and outer layer 50exhibit different coefficients of thermal expansion. For example, theinclusion of second intermediate layer 48 having multiple sublayers thatare compositionally graded may reduce the coefficient of thermalexpansion gradient, or in other words, make the compositional transitionfrom the first intermediate layer 46 to outer layer 50 more gradual,thus making the change of coefficients of thermal expansion moregradual. It may be understood that the more sublayers included in thesecond intermediate layer, the lower the interfacial stresses due tomismatches of coefficients of thermal expansion. The number ofsub-layers in the transitional layer need not be limited, but may bechosen according to the desired properties of the article and the timeand expense involved in producing the article.

Furthermore, a coating may also include a second intermediate layer thatis not divided into sub-layers, but which includes a continuously gradedcomposition. For example, the second intermediate layer may becompositionally most similar to the first intermediate layer at thefirst intermediate layer-transitional layer interface, and most similarto the outer layer at the outer layer-transitional layer interface, witha composition that continuously transitions from the first intermediatelayer composition to the outer layer composition along the depth of thetransitional layer.

In some embodiments, intermediate layer 48 may be a rare-earth silicatelayer that transitions into another rare-earth silicate layer. Forexample, intermediate layer 48 may include Yb-disilicate while outerlayer 50 may include Yb-monosilicate. The respective layers may bedeposited as discrete layers or as functionally graded materials.

Some embodiments of the disclosure may relate to a method of coating asubstrate consisting essentially of depositing mullite on the substratevia thermal spraying to form a first layer; and depositing a secondmaterial on the first layer to form a second layer, wherein the mullitecomprises mullite powder formed via at least one of a fused plus crushor sinter plus crush process.

Some embodiments of the disclosure may relate to a method of coating asubstrate consisting essentially of depositing silicon on the substrateto form a silicon layer, depositing mullite on the silicon layer viathermal spraying to form a first layer; and depositing a second materialon the first layer to form a second layer, wherein the mullite comprisesmullite powder formed via at least one of a fused plus crush or sinterplus crush process. Such a method may not include depositing the mullitelayer at a relatively high temperature and/or heat treating the mullitelayer, as previously described. In some examples, the method may furtherconsist essentially of heat treating the silicon layer and substrateprior to depositing the mullite on the silicon layer.

Example

FIGS. 5A and 5B are cross-sectional photographs of non-leading edgeportions of example articles 52 a and 52 b, respectively.

FIGS. 6A and 6B are cross-sectional photographs of a leading edgeportion of example articles 52 a and 52 b, respectively.

Referring to FIGS. 5A and 6A, article 52 a included EBC 54 a provided onceramic matrix composite substrate 56 a. EBC 54 a included silicon bondlayer 58 a, intermediate layer 60 a, and outer layer 62 a. Silicon bondlayer 58 a was formed by depositing silicon directly on the surface ofsubstrate 56 a via plasma spraying. Intermediate layer 60 a was formedby depositing mullite powder that was produced via a fused plus crushprocess via air plasma spraying on silicon bond layer 58 a. Outer layer62 a was formed by depositing ytterbium di-silicate on intermediatelayer 60 a. Notably, each layer 58 a, 60 a, 62 a was formed bydepositing the respective layer material via air plasma spraying whilethe substrate was held in a furnace at a temperature of approximately1200 degrees Celsius. Article 52 a was not heat treated after EBC 54 awas applied to substrate 56 a.

Referring to FIGS. 5B and 6B, article 52 b included EBC 54 b provided onceramic matrix composite substrate 56 b. EBC 54 b included silicon bondlayer 58 b, intermediate layer 60 b, and outer layer 62 b. Silicon bondlayer 58 b was formed by depositing silicon directly on the surface ofsubstrate 56 b via plasma spraying. After deposition, silicon bond layer58 b underwent diffusion heat treatment at approximately 1225 degreesCelsius (+/−25 degrees) for approximately 1 hour. Intermediate layer 60b was then formed by depositing mullite powder that was produced via afused plus crush process via air plasma spraying on silicon bond layer58 b. Outer layer 62 b was formed by depositing ytterbium di-silicate onintermediate layer 60 b. Unlike article 52 a of FIG. 5A, each layer 58b, 60 b, 62 b was formed by depositing the respective layer material viaair plasma spraying while substrate 56 b was initially at a temperatureless than approximately 50 degrees Celsius. In particular, the processof air plasma spraying of the layer material on substrate 56 b wasinitiated when substrate 56 was at approximately 25 degrees Celsius,which was the ambient temperature of the room in which the substrate waslocated in at the time the process was carried out. Similar to that ofarticle 52 a, article 52 b was not heat treated after EBC 54 b wasapplied to substrate 56 b.

Articles 52 a and 52 b were exposed to substantially the same steamthermal cycling in an environment of greater than 1800 degreesFahrenheit and greater than 60% humidity. The steam thermal cyclinginvolved exposing articles 52 a and 52 b to the described environmentfor greater than 100 cycles for a total of amount of time greater than100 hours. Additionally, articles 52 a and 52 b were exposed to laserheat flux through EBCs 54 a and 54 b, respectively, above 1500 degreesFahrenheit in conjunction with the steam thermal cycling, for greaterthan 5 hours. Such testing of articles 52 a and 52 b simulated engineoperating conditions in a high temperature combustion environment in thepresence of water vapor.

The cross-sectional photographs of FIGS. 5A, 5B, 6A and 6B were takenafter articles 52 a and 52 b, respectively, underwent theabove-described steam thermal cycling. As illustrated by the photographsof FIGS. 5A and 5B and FIGS. 5A, 5B, 6A and 6B, article 52 b exhibitedequivalent durability to that of article 52 a. In particular, layers 58a, 60 a, and 62 a of EBC 54 a, as well as layers 58 b, 60 b, and 62 b ofEBC 54 b were substantially crack-free after steam thermal cycling andmaintained excellent adherence to substrates 52 a and 52 b,respectively. Additionally, the oxidation of substrates 52 a and 52 bwas minimal, as evidenced by the lack of oxide scale formed between EBC54 a, 54 b and substrates 52 a, 52 b, respectively, indicating that EBCs54 a, 54 b provided excellent environmental protection for articles 52a, 52 b, respectively. It was observed that that the room-temperaturesprayed EBC (EBC 54 b) was more tolerant to mechanical strains than thefurnace sprayed EBC (EBC 54 a), which likely may be due to thecomparatively higher porosity and microcracks in the room temperaturesprayed EBC.

Various embodiments of the invention have been described. These andother embodiments are within the scope of the following claims.

1. A method of coating a substrate, the method comprising: depositingmullite on the substrate during a first time period via thermal sprayingto form a first layer, the mullite comprising mullite powder formed viaat least one of a fused plus crush or sinter plus crush process; anddepositing a second material on the first layer to form a second layer,wherein the substrate is at a temperature less than approximately 50degrees Celsius at approximately a beginning of the first time period.2. The method of claim 1, wherein the temperature of the substrate isless than approximately 40 degrees Celsius at approximately thebeginning of the first time period.
 3. The method of claim 1, furthercomprising maintaining the substrate at a temperature less thanapproximately 200 Celsius throughout the first time period.
 4. Themethod of claim 1, wherein the first layer comprises crystalline mullitesubstantially immediately following the deposition of the mullite. 5.The method of claim 1, further comprising depositing silicon on thesubstrate to form a silicon layer before depositing the mullite on thesubstrate.
 6. The method of claim 5, further comprising heat treatingthe silicon layer prior to depositing the mullite.
 7. The method ofclaim 6, wherein heat treating the silicon layer comprises heat treatingthe silicon layer at a temperature of about 800 degrees Celsius to about1250 degrees Celsius for about 0.2 hours to about 4 hours.
 8. The methodof claim 1, wherein the second layer comprises at least one of bariumstrontium aluminum silicate (BSAS), ytterbium mono-silicate or ytterbiumdi-silicate.
 9. The method of claim 1, wherein the first layer has athickness of approximately 1 mil to approximately 6 mils.
 10. The methodof claim 1, wherein the second layer has a thickness of approximately 2mils to approximately 15 mils.
 11. The method of claim 1, wherein thesubstrate is not heated via a furnace during the deposition of themullite in a manner that substantially raises the substantially uniformtemperature of the substrate in conjunction with the deposition of themullite.
 12. The method of claim 1, wherein thermal spraying comprisesplasma spraying.
 13. The method of claim 1, further comprisingdepositing silicon on the substrate to form a silicon layer beforedepositing the mullite on the substrate, wherein the silicon layerthickness is approximately 0.5 mils to approximately 5 mils.
 14. Amethod of coating a substrate, the method comprising: depositing siliconon the substrate to form a silicon layer; heat treating the siliconlayer; and depositing mullite on the silicon layer via thermal sprayingto form a first layer subsequent to the heat treatment of the siliconlayer, the mullite comprising mullite powder formed via at least one ofa fused plus crush or sinter plus crush process.
 15. The method of claim14, wherein the mullite is deposited on the silicon layer during a firsttime period and the substrate is at a temperature less thanapproximately 50 degrees Celsius at approximately a beginning of thefirst time period.
 16. The method of claim 14, wherein the heattreatment comprises heat treating the silicon layer at a temperature ofabout 800 degrees Celsius to about 1250 degrees Celsius for about 0.2hours to about 4 hours.
 17. The method of claim 14, further comprisingdepositing a second material on the first layer to form a second layer,the second layer comprising at least one of at least one of bariumstrontium aluminum silicate (BSAS) or a rare-earth silicate.
 18. Themethod of claim 14, further comprising: depositing a second material onthe first layer to form an intermediate layer; and depositing a thirdmaterial on the intermediate layer to form a third layer.
 19. The methodof claim 18, wherein the intermediate layer comprises approximately 20to approximately 50 weight percent mullite and at least one of ytterbiumdi-silicate and ytterbium mono-silicate, and the third layer comprisesone of ytterbium di-silicate and ytterbium mono-silicate.
 20. The methodof claim 18, wherein the intermediate layer comprises ytterbiumdi-silicate and the third layer comprises ytterbium mono-silicate.